1. Field of the Invention
The present invention relates to a technique for generating a cyclic signal having a sloping waveform.
2. Description of the Related Art
Switching power supply devices such as a switching regulator or the like employ a pulse signal in order to switch a switching element ON and OFF. As a method for generating such a pulse signal, a technique is employed in which the charging operation and the discharging operation of a capacitor are repeatedly alternated so as to generate a cyclic signal (triangle wave signal) having a sloping waveform, and the cyclic signal thus generated is sliced at a predetermined level so as to generate a pulse signal based upon the points of intersection of the cyclic signal and the predetermined level (Patent documents 1 and 2).
As an oscillator circuit configured to generate a triangle wave signal, a self-running method (self-excitation method) and an external synchronization method (separate excitation method) are known. With the former method, the voltage at a capacitor is compared with a predetermined threshold voltage, and the timing at which the capacitor is charged and discharged is controlled with the timing obtained based upon the comparison result. On the other hand, with the latter method, the timing at which the capacitor is charged and discharged is controlled with the timing of an external clock signal used as a reference signal. This method is effectively employed in a case in which it is desired that an oscillator circuit operate synchronously with an external circuit.